My ram1500,4.7 heats when powering but if I go to neutral gauge drops back quikly.changed thermostas,water pump,rad and cover,bypassed heater core.nothing in oil,does not use coolant,still blows heat from blower. Top rad hose is hot but bottom one is cool.HELP

You may have corrosion built up inside the engine block, preventing proper coolant flow throughout the system.

You may have corrosion built up inside the engine block, preventing proper coolant flow throughout the system.

Last time I drained system 1 week ago coolant was clean as a whistle